If you watch a lot of electronics videos on YouTube, you’ve probably come across the guy with the Swiss accent. Andreas is a big fan of LoRa and its IoT applications, but it was this video on Meshtastic that really intrigued me. A license-free mesh network with very low power, crazy long range, and inexpensive hardware? Given that I’m still struggling a bit with The Things Indoor Gateway (and finding that TTN may not be the best LoRaWAN provider for a project I’ve had in mind), I thought Meshtastic might be a neat way to play more with LoRa. Sure enough, there’s loads of things you can do with Meshtastic, the development is continuing rapidly, and the community around the project is active and helpful.
Currently (though this may change soon) the favored hardware for Meshtastic is the TTGO T-Beam v1.1, as it offers all the bells and whistles of GPS, Bluetooth, Wi-Fi, and OLED display. However, the device’s layout and form factor is… …awkward. Thankfully, there’s a higher-end version of the board that drops the soldered-on SMA antenna connector, has an improved GPS module, and features the much more capable SX1262 transceiver chip (which can support Semtech’s upcoming FHSS modulation scheme in addition to the existing ‘chirp’ based modulation).
Despite the already excellent battery life (several days) provided by a 18650 cell, the width of the T-Beam board made me think that accommodating a 26650 cell shouldn’t bulk up the entire assembly too much. While 26650 cells aren’t nearly as common as the 18650 size, they should be able to offer double the energy capacity based on volume (in theory, that is – more on this in a moment). Even if a 26650 cell isn’t readily available, a 18650 can be sleeved to make it fit a 26650 holder (the lengths are the same). Finally, I decided to go with a better GPS antenna, as the stock one would need relocating anyhow due to the larger battery holder. My purchase list looked essentially like this:
TTGO T-Beam v1.1 with SX1262 and NEO-M8N (I used the 915MHz version for US operation – check your country’s bandplan for the appropriate ISM band to use)
BT-25A 28x28x7 ceramic GPS antenna (bigger is better, but this was about as large as practical)
26650 battery holder (make sure to get one with an ‘open’ bottom so that the cell can sit right up against the PCB for the lowest profile just like the stock 18650 holder)
0.96″ 4-pin I2C OLED display (note that I bought the one labeled as ‘new’, as it provides a slightly smaller footprint)
M2 button head stainless fastener set (I could have also just bought the fasteners individually from McMaster-Carr, but I wasn’t sure what lengths I’d need, and having an assortment on-hand always proves useful)
4-pin Dupont connector cable (I actually used a scrap cable from the parts bin instead)
4″ U.FL to SMA RG-178 coax pigtail (yes, the SX1262 T-Beam includes an antenna pigtail already, and upgrading to RG-178 coax is basically chasing nanovolts, so this is entirely frivolous and I should feel shame)
The only other component needed was a 26650 cell. Amazon doesn’t have very much, so I found these on AliExpress. The Project Farm review of 18650 cells has made me wary of whatever a battery label claims, so I bought the same type of battery tester as Todd used. Once the GTF cells finally arrived (taking longer than perhaps any other AliExpress purchase I’ve made), I dropped a pair of them into the tester, let it chug overnight with the capacity test, and found that they actually have a feeble rating of only 2600-2800 mAh. Less than the roughly 3200 mAh I’m getting on these JOUYM 18650 3500 mAh cells (my current favorite for Li-Ion cells). So the search for good 26650 cells continues.
With all of the components in hand, I started modeling an enclosure for everything. Rather than try to model the PCB on my own, I found a v1.1 update by ‘DJ’ of a v0.7 T-Beam board by Muchammad Sobrun Ibnu Atfal that only needed a little tweaking. I slapped a cylinder on the back for a 26650 cell, and then just started drawing around it. I wish I could say I had a grand vision in mind, but the design just grew organically as I figured out where to place the LoRa antenna, GPS antenna, and OLED display. Note that it was designed to print with minimal supports – print the top and OLED cover face-down, and the bottom shell bottom-down (the bottom only needs supports for the antenna hole and the GPS pack).
Assembling everything was pretty straightforward. First step was to desolder the stock 18650 battery holder, then solder the 4-pin cable for the OLED display to the T-Beam board. Note that unlike the display that is included with the lower cost SX1272/NEO-6M T-Beam, the one I sourced has the Vcc and GND pins reversed, so it can’t be soldered directly to the board anyhow. After soldering the cable, I clipped the leads poking out on the backside as close to the PCB as possible (since the new battery holder would be right up against the PCB board where the solder pads are). Securing the leads with a bit of ‘hot snot’ (hot glue gun) finished up the top side.
Then I removed the ‘ears’ on the sides of the 26650 battery holder via the judicious application of a belt sander (though a side cutters and utility knife would do as well). I also relieved a little bit of the bottom surface of the holder right where it would sit over the OLED cable solder pads. After bending the battery terminal tabs down a little and tinning them, I soldered the 26650 holder to the PCB in the same spot as where the 18650 holder had been.
Finally, I ever-so-gently smashed the pins on the OLED display 90 degrees over for clearance.
Everything plopped into the printed case quite snugly – looks like I got everything just right. I did drill out the corner holes on the top and the OLED cover with a 5/64″ drill bit, and let the 2mm screws self-tap themselves into the bosses on the bottom shell and the top plate. I also drilled out the holes on the bottom shell for the 2mm captive nuts that can be used for attaching accessories like a lanyard loop, belt clip, flamethrower, etc.
Turns out that I had plucked the yellow OLED display from the assortment that I ordered, and I have to say it looks downright proper with the black enclosure (though readability in sunlight is poorer than with the white OLED included with the 1272 T-Beams). Files for the enclosure are here:
Addendum1: I purchased these UltraFire ‘6800mAh’ 26650 cells on Ebay, hoping that the lower rating might hint at a more honest appraisal of the cell’s capacity. Nope, each cell measured at an utterly abysmal 1244mAh. If anyone knows of a 26650 cell that does not suck (or at least offers honesty in labeling), please leave a comment.
Addendum2: After reflecting on the fact that the lousy GTF and UltraFire cells have their branding pre-printed on the cell wrap, while the really good JOUYM cells have continuous inkjet coding on the cell wrap, perhaps indicating that the cells have been tested and ‘binned’, then marked afterwards. So I looked around on AliExpress for 26650 cells that were also marked in the same fashion, and found these nondescript Tewaycell 5000mAh units. Since Tewaycell looked to manufacture a variety of battery types, I guessed they’d probably be more honest about capacities, since their sales will overwhelmingly be to other manufacturers. I purchased a pack of two and tested them as soon as they arrived. I was absolutely floored – not only was the capacity as much as advertised, it was well over! One cell measured 5676mAh and the other a whopping 5775mAh! Well done, Tewaycell!